One type of TV receiver is equipped with an auxiliary or EIA input connector for receiving an auxiliary video signal (for example, computer graphics signal, teletext signal, etc.) in addition to receiving off-the-air broadcast TV signal applied to its antenna terminals. Both off-the-air and auxiliary video signals are capacitively coupled to a signal selection network, which applies one of the two input signals to an image display device or a kinescope in response to a control signal.
The signal selection network includes output clamping circuits, responsive to a preset reference potential and coupled to the respective signal paths, for automatically establishing a common black image or brightness level for the video signals applied to the kinescope. The clamping circuits work well as long as the black image component reference levels of the incoming video signals applied to the signal selection network are held substantially constant. Any fluctuations in the black image reference levels of the incoming signals may cause corresponding variations in the brightness levels of the pictures displayed on the TV screen.